The life of a rechargeable battery system in a single discharge cycle is difficult to predict because it is affected by many factors. Age, operating temperature, discharge conditions, and battery chemistry are some of the major factors. Though voltage monitoring is the easiest and most commonly used method of determining a rechargeable battery's remaining capacity in a single discharge cycle, different battery chemistries provide different voltage curves under different conditions. FIG. 1 shows two typical voltage curves of a lithium-ion rechargeable battery under different operating conditions. The solid line is a voltage curve under an optimal operating condition, i.e., lower discharge rate, higher operating temperature, and a newer cell. The dashed line is a voltage curve under a worse operating condition, i.e., higher discharge rate, lower operating temperature, and an older cell. As can be seen, in both curves, the voltage is relatively constant during the useful life of a battery's single discharge cycle; it drops off rather suddenly at the end of the cycle.
Since the voltage drop is small just prior to system failure 103, it is difficult to predict where the actual point of failure will be during a discharge cycle. Therefore, a battery system may either be cut off too soon or fail unexpectedly. The latter may result in inconvenient or serious consequences. For example, in a computer device, an unexpected battery failure may result in loss of important data; in an electric vehicle, a motorist may be stranded; and in a medical device, it could be a matter of life and death.